The invention relates generally to sensing ground faults in motor drives. Particularly, this invention relates to an integrated common mode choke for sensing ground faults.
Motor drives often include converter circuitry for converting incoming AC power into DC power, and inverter circuitry for converting the DC power into controlled frequency AC power output. Output currents for three phases of the AC power may be sensed for motor control and protection. For example, a ground current may be determined by summing all three measured phase currents. When the sum of the three phase currents exceeds a threshold value (e.g., 20% of the rated current for the motor drive), a ground fault of the motor drive may be considered to have occurred. Often, firmware-based ground fault detection samples the sum of the three phase currents, filters the result, and generates an alarm when the threshold value is exceeded. The threshold value may be adjustable to low levels, but this method of ground fault detection is still somewhat sensitive to noise and feedback accuracy. Furthermore, the ground fault current may often be very low on AC power systems that are not solidly grounded (e.g., lower than 10 amperes) and, thus, the motor drive may not fault when it perhaps should. As such, the firmware-based ground fault alarm method does not reliably detect ground faults of the motor drive. Therefore, there is a need for more reliable detection of low levels of ground fault currents for motor drives.